For words that have only three letters, there is absolutely no 4th letter. As an example with the word “include”, there are no more letters Later on and thus must be the word "add". The word “addict” is from the BIP39 wordlist however you would have utilised “addi” for “addict”. https://top10bookmark.com/story18779596/not-known-factual-statements-about-bip39-explained